The field of the invention is apparatus and methods for measurement of volumetric flow rate of flowing fluid. More particularly, the invention relates to dynamic or inferential measurement devices, as opposed to positive displacement measuring devices, which former devices are additionally of fluidic operation.
Conventional fluidic measuring devices in the field to which the present invention relates are disclosed in the following U.S. Pat. Nos.
______________________________________ U.S. Pat. No. ISSUED INVENTOR(S) ______________________________________ 3,690,171 12 September 1972 Tippetts, et al 3,889,534 17 June 1975 J. Grant 4,050,304 27 September 1977 A. Thomas 4,107,990 22 August 1978 C. Ringwall 4,404,859 20 September 1983 Ohsawa, et al ______________________________________
These teachings establish that the possibility of measuring fluid flow rate, either on a volume or mass basis, has been recognized for a number of years. The first three teachings listed above are believed to apply wall-attachment type fluidic oscillators to, respectively, measurement of volumetric flow rate as a function of oscillator frequency, to measurement of mass flow rate as a function of oscillator amplitude and frequency, and to measurement of mass flow rate as a function of oscillator frequency and total pressure drop across the oscillator.
The Ringwall U.S. Pat. No. 4,107,990 recognizes the deficiencies of wall-attachment fluidic oscillators in the application to flow rate measurement. Accordingly, the Ringwall patent teaches use of a differential pressure proportional fluidic oscillator to provide a volumetric flow rate meter. However, to extend the measurement range of the Ringwall teaching beyond that obtainable with a single oscillator, multiple amplifier stages must be utilized. Additionally, it is believed that the signal quality which is obtained from an oscillator as taught by Ringwall may be less than optimum. This output signal is believed to be contaminated with dynamic oscillation, or ringing, noise which is internally self-generated by the oscillator as a result of internal fluid inductances and capacitances.
An alternative approach to fluid mass flow rate measurement is presented by U.S. Pat. No. 4,508,127 issued 2 April 1985 to a coinventor of the present invention and assigned in common therewith. The disclosure of the '127 patent is specifically incorporated herein by reference to the extent necessary for a complete disclosure and understanding of the present invention. The '127 patent teaches use of a dynamic volumetric flow rate meter, such as a turbine meter, to obtain a signal indicative of volumetric fluid flow rate. A fluidic oscillator having a regulated total pressure drop thereacross is employed to generate a second signal indicative of fluid density. The two signals are combined by multiplication to obtain an indication of fluid mass flow rate.
However, it is desirable to provide a volumetric fluid flow rate meter which avoids the use of moving-part type flow meters. The fluidic oscillator because of its rugged, no-moving-parts construction is recognized as offering considerably improved service life over all flow meters having moving parts, such as the turbine flow meter, for example. Further, the limited flow measurement range and need for multiple oscillators of the Ringwall teaching should be avoided. Finally, it is highly desirable to provide such a volumetric flow rate meter with a "clean" output signal substantially free of both self-generated noise or ringing, and noise of electrical origin.